>And why don't you just use a margin-left of 10, so you haven't the margin-right problem?

If I do that, I will not be able to align the content with the rest of the site on the left.

Usually, people would go with centered at margin-right 5 and margin-left 5. So, at least, that would diminish the gap on the right. But, while it may look good when you have a specific number of items on each row, if you end up with only one item on the last row, that one would fall by itself in the middle and that would break the alignment.